Digital telephone sytems are used to switch PCM encoded voice signals and computer or other data signals between peripheral units. These peripheral units can be standard analog telephones connected by CODECS (coder-decoders), digital telephones, computer terminals, tone generators, etc. The peripherals may be "dumb", that is, have passive or non-existent controllers, or may be "smart", containing their own controllers.
The telephone system typically is formed of a main controller which communicates with switching arrays which switch time divided multiplexed signals between peripherals.
In order to reduce the time overhead of the main controller, all or groups of the peripherals are often controlled by a peripheral controller, usually formed of a microprocessor with auxiliary circuitry. The main controller communicates with the peripheral controller which itself controls the peripherals. Activities undertaken by the peripheral controller are the regular scanning of scan points in the peripherals, the transmission of control signals to the peripherals and the transmission of supervisory signals from the peripherals to the main controller.